There is no happiness, peace or tranquility similar to that caused by monotheism. Allah says: {It is those who believe (in the Oneness of Allâh and worship none but Him Alone) and confuse not their belief with Zulm (wrong i.e. by worshipping others besides Allâh), for them (only) there is security and they are the guided.}(Al-An’âm:82)Thus, the more one’s monotheism is completed and perfected, the more he feels secure, tranquil and happy in this world and the Hereafter, because Allah widens his chest and fills it with pleasure and relief. On the other hand, polytheism - Allah forbid –brings misery and distress into one’s chest, as if he was ascending into the sky. Allah says: {And whomsoever Allâh wills to guide, He opens his breast to Islâm, and whomsoever He wills to send astray, He makes his breast closed and constricted, as if he is climbing up to the sky. Thus Allâh puts the wrath on those who believe not.}(Al-An’âm:125)
source:
https://www.path-2-happiness.com/en/...s-of-happiness